Method of hot-shaping and hardening a sheet steel blank

ABSTRACT

A sheet steel blank is heated to the austenite range and formed in a cooled tool pair that rapidly cools the formed product until the product&#39;s temperature drops somewhat below the temperature Ms for the start of the formation of martensite. The cooling is rapidly interrupted and the product&#39;s temperature is raised until it exceeds Ms and is maintained there until the material comes to contain more than 50% by volume bainite. The short time under the Ms temperature favours the formation of bainite and shortens the holding time.

FIELD OF THE INVENTION

The invention relates to a method of hot forming and hardening a sheetsteel blank by forming the material heated to the austenite range in atool pair that rapidly cools the shaped product.

BACKGROUND OF THE INVENTION

In the automotive industry products of high strength steel are beingused more and more that are formed and hardened with press-hardeningtechnology, that is, a sheet steel blank of hardenable boron steel isheated to the austenite range and is formed in a cooled tool pair andmaintained in the tool pair with the tool pair as a fixture for severalseconds so that the formed product obtains a martensitic structure. Thisprocess yields a tensile strength of above 1400 MPa. The subsequentworking, e.g., punching or laser cutting of edges is made on hardenedmaterial but sometimes a rapid cooling of portions that are to besubsequently worked is prevented in order to avoid the working of fullyhardened material.

A steel with bainite structure can have approximately the same hightensile strength as the one obtained with the press-hardening processbut the bainite structure is tougher and more ductile than themartensitic structure. U.S. Pat. No. 6,149,743 describes a method forbainite hardening. The material is rapidly cooled from the austeniterange to just above Ms (the temperature for the start of the formationof martensite) and this temperature is maintained for approximately 10hours and, toward the end of the holding time, the temperature is raisedin order to shorten the hardening time. This process can be used forroller bearings.

PURPOSE OF THE INVENTION

It is an object of the invention to produce, in an economical manner,products that have better material properties than those that areproduced with conventional press hardening. Another object is to createthe possibility of subsequent working before the product fully hardens.

BRIEF DESCRIPTION OF THE INVENTION

The formed product is rapidly cooled until the product's temperaturedrops below the temperature for the start of the formation of martensiteand the product's temperature is then raised until it exceeds thetemperature for the start of the formation of martensite and ismaintained there until bainite is formed. The times and the temperaturesfor the formation of martensite and the formation of bainite can beselected so that the material comes to contain more than 50% by volumebainite. The forming and the rapid cooling take a few seconds whilemaintaining the heat takes a few minutes. The time for maintaining theheat (the holding time) can be utilized for subsequent working, e.g.edge cutting.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a time-temperature diagram for an example of a process inaccordance with the invention.

FIG. 2 is a block diagram that shows the process.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 2 shows a block diagram with an austenitic phase in a furnace 11, aforming phase in a cooled forming tool 12, and a subsequent temperatureholding in a furnace 13.

The temperature curve in FIG. 1 for an example of a process inaccordance with the invention shows a first part 20 that is the heatingin a furnace of a blank to the austenite range. The moving of thematerial to a cooled forming tool pair (curve part 21 in the temperaturecurve) normally takes 5-10 seconds and entails a small reduction oftemperature. The forming in the cold tool pair is in the order ofmagnitude of one or a few seconds but the product must remain in thetools until it is rapidly cooled to somewhat below Ms (the temperaturefor the start of a formation of martensite). The formation of martensiteis instantaneous and is a function of the temperature but is not afunction of the time. This cooling is accordingly carried out with thetool pair as fixture. The forming and rapid cooling is designated as 22.The product is rapidly taken out of the tool pair and the startedformation of martensite generates heat and entails an elevation oftemperature 23 that may exceed Ms. Additional heat can be used, e.g.,induction heat or radiant heat in order to rapidly bring the temperatureup above Ms. The product is then moved to a furnace where the product ismaintained at a rather even temperature somewhat above Ms for a numberof minutes represented by the curve part 24. The product is thereaftercooled down in the air, which is represented by the dashed curve part 25or is speeded up as is represented by the curve part 26.

The start of formation of martensite favours the formation of bainiteand the holding time for the formation of bainite is shortened,generally to less than half an hour or below 10 minutes or even below 1min. This is a prerequisite for an economical process The amount ofmartensite can be predetermined by selecting how much the temperature islowered below Ms. The temperature for the formation of bainite can bemade to vary by a few tens of degrees and therefore the holding time canbe used for subsequent working, e.g. edge cutting, which is advantageousto carry out before the material reaches its full hardness, since thisreduces the wear on the tool and also reduces the risk for theinitiation of fissures and the following formation of fissures when theproduct is used. The subsequent working can also be integrated into theshaping tool and be integrated in the curve part 22, i.e. it can becarried out before or during the formation of martensite.

In a modified process the product can be formed in a first tool pair andcooled down to just over Ms and the formed product can then be moved toa second tool pair that makes the fixture and cools the product down tosomewhat below Ms. This second tool pair can carry out the subsequentwork at the same time, e.g. edge cutting, before the product is moved tothe furnace for maintaining the heat. Alternatively, the subsequentworking can be integrated in the first tool pair. The two tool pairs canwork simultaneously, which shortens the cycle time.

Boron steel is used in conventional press hardening, that is, acarbon-manganese steel with boron, that completely hardens tomartensite. A carbon-silicon-manganese steel can be suitably used in aprocess in accordance with the invention. The steel can have a carboncontent of 0.2-0.3 weight %, a manganese content of 1-2 weight % and asilicon content of 1-2 weight %. In addition, chromium and othercustomary alloy substances with a total content less than 1 weight % maybe present. Silicon prevents the separation of cementite and creates thedesired microstructure.

1. A method of hot shaping and hardening a sheet steel blank by formingthe blank, heated to the austenite range, in a tool pair that rapidlycools the formed product, characterized in that the formed product iscooled until the product's temperature drops below the temperature forthe start of the formation of martensite, Ms, and the product'stemperature is then raised until it exceeds the temperature for thestart of the formation of martensite and is maintained hot so thatbainite is formed.
 2. The method according to claim 1, characterized inthat the raised heat is maintained for a holding time of 1-20 minutes.3. The method according to claim 1, characterized in that temperaturesare selected so that the material comes to contain more than 50% by vol.bainite.
 4. The method according to claim 1, characterized in that theformed product is subsequently worked directly in the tool pair.
 5. Themethod according to claim 1, characterized in that the material iscooled and shaped in a first tool pair down to a temperature exceedingMs and the shaped blank is then moved to a second tool pair and cooledfurther to a temperature below Ms with this tool pair as a fixture. 6.The method according to claim 5, characterized in that the formedproduct is subsequently worked in the first tool pair.
 7. The methodaccording to claim 5, characterized in that the formed product issubsequently worked in the second tool pair.
 8. The method according toclaim 2, characterized in that the formed product is subsequently workeddirectly in the tool pair.
 9. The method according to claim 3,characterized in that the formed product is subsequently worked directlyin the tool pair.
 10. The method according to claim 2, characterized inthat the material is cooled and shaped in a first tool pair down to atemperature exceeding Ms and the shaped blank is then moved to a secondtool pair and cooled further to a temperature below Ms with this toolpair as a fixture.
 11. The method according to claim 3, characterized inthat the material is cooled and shaped in a first tool pair down to atemperature exceeding Ms and the shaped blank is then moved to a secondtool pair and cooled further to a temperature below Ms with this toolpair as a fixture.
 12. The method according to claim 4, characterized inthat the material is cooled and shaped in a first tool pair down to atemperature exceeding Ms and the shaped blank is then moved to a secondtool pair and cooled further to a temperature below Ms with this toolpair as a fixture.
 13. The method according to claim 10, characterizedin that the formed product is subsequently worked in the first toolpair.
 14. The method according to claim 11, characterized in that theformed product is subsequently worked in the first tool pair.
 15. Themethod according to claim 12, characterized in that the formed productis subsequently worked in the first tool pair.
 16. The method accordingto claim 10, characterized in that the formed product is subsequentlyworked in the second tool pair.
 17. The method according to claim 11,characterized in that the formed product is subsequently worked in thesecond tool pair.
 18. The method according to claim 12, characterized inthat the formed product is subsequently worked in the second tool pair.